You need to alter the postcodes from which you accept PRIME 1-day and 2-day orders.
Preferred carriers - so for example we chose only DPD UK, which meant all orders from places like Northern Ireland are standard shipping, and then we can use any approved courier (there is a list, but includes, Hermes, DHL, Yodel, Parcelforce, Royal Mail, etc).
If you choose only Amazon Shipping, then that should mean all postcodes they do not cover you will be ok for
If due to size of parcel, then you should not be choosing PRIME as an option, or you need to have DPD, or Royal Mail set up to take those parcels. Sell those as non Prime and then you can ship using any courier
For DPD issues Number: 0121 336 4900 Option 2 - I’ve email addresses as well, but the Amazon Manager should have given you those if you passed everything to use DPD